Is there a list for all available python functions?
These functions (or rather methods, properties and attributes) you are asking for are blender-specific, thus part of the Blender Python API (a.k.a. bpy) - just to clear it up. For standard python module features, please refer to the official python docs.

.polygons, .length etc. are properties of certain Blender type instances (objects). To find out what they do and which features are available, you need to check out the bpy API docs for a particular type (class).

There is a page in the docs about how to use the docs:
Blender/Python API Reference Usage: examples of how to use the API reference docs


  Example
  
  .polygons is a property of the type bpy.types.Mesh.
  
  It is a type on its own, MeshPolygons, > which has an .active property and an add() method.
  
  If you use the slice notation on .polygons, you will get elements returned of a bpy_prop_collection. The element type is MeshPolygon. A MeshPolygon has i.e. a .normal property in turn.


As you can see, the methods and properties are not found in a single place. The most valuable information is hidden in the bpy.types section of the docs. You can use the online search, or use Blender's built-in Python Console with auto-complete to figure out what is where. You can also find out the type and search for it in the API docs by adding bl_rna.identifier to an object:

&gt;&gt;&gt; bpy.context.object.data.bl_rna.identifier
'Mesh'

&gt;&gt;&gt; bpy.context.object.data.polygons.bl_rna.identifier
'MeshPolygons'

&gt;&gt;&gt; bpy.context.object.data.polygons[0].bl_rna.identifier
'MeshPolygon'


Prepend bpy.types. and you have your type to lookup in the docs!